Electrochemical capacitors have been developed in recent years. The electrochemical capacitors include, in its category, an electric double-layer capacitor (EDLC) that utilizes capacitance formed by electrostatically storing positive and negative charges at interfaces between electrodes and an electrolytic solution, and a redox capacitor that utilizes capacitance that is stored along an electron transfer process (Faraday process) on an electrode surface.
As an electrolyte of the redox capacitor, an acid aqueous solution such as sulfuric acid or hydrochloric acid, cesium hydrogensulfate, or the like is used (see Patent Document 1 and Patent Document 2).